Re: [Boost-users] How to make multi_index_container work withstd::unique_ptr?

On Wed, Dec 21, 2011 at 4:54 AM, Igor R wrote:

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And what's wrong with just shared_ptr<B>? Why do you need the additional level of indirection?

For example

int main() { shared_ptr<B> p(new D1); shared_ptr<B> user1 = p; // User1 uses p p.reset(new D2); // p changes pointed type to D2, user1 still points to D1 return 0; }

or

int main() { shared_ptr<B> p(new D1); shared_ptr<B>* user2 = &p; // User2 uses p, but the p.use_count() is still 1. p.reset(new D2); // p changes pointed type to D2, *user2 points to D1 return 0; }

Ok, so you actually want the following: 1) The container itself shouldn't have "strong" references to your objects 2) An object can be recreated/replaced when it doesn't have any strong references. Am I right?

If so, store weak_ptr<Base> in your container:

// pseudo-code int main() { multi_index_container< weak_ptr<B>, // some indices

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container; shared_ptr<B> p(new D1); container.insert(p);

// at some futher stage: auto iterator = container.find(someKey); auto p = iterator->lock(); // if the weak_ptr expired, i.e. no other clients use object, you may re-create it if (!p) recreateIt(); }

Hi, Igore, I'd like to first thank you for keeping helping me one this problem. I'd like to try to make my requirements clear by some pseudo codes. int main() { multi_index_container< shared_ptr, ..named_index..

container; // container own the objects.

// Insert object M of type D1 into container container.insert(shared_ptruse_count() == 1) { container.erase("M"); } // Replace object M in the container to N of type D2. user use object N now since it share info with container. itorator it = container.find("M"); shared_ptr& ppb = *it; (*ppb).reset(new D2("N")); container.replace(it, ppb); } I am look for simple ways (not double pointer) to implement those requirements. Best regards, Jayden